Semiconductor devices for detecting the physical quantity distribution in which a plurality of unit elements (for example, pixels) responsive to electromagnetic waves, such as light or radiation, input from an external source, are disposed in a line or a matrix are used in various fields.
In the video equipment field, CCD (Charge Coupled Device), MOS (Metal Oxide Semiconductor), or CMOS (Complementary Metal-oxide Semiconductor) solid-state imaging devices for detecting light (an example of electromagnetic waves) as the physical quantity are used. Such imaging apparatuses read the physical quantity distribution obtained by converting light into an electric signal by using the unit elements (pixels in the solid-state imaging devices) as the electric signal.
Solid-state imaging devices include amplifying solid-state imaging devices. Amplifying solid-state imaging devices have pixels formed of amplifying solid-state imaging devices (APS; Active Pixel Sensors/also referred to as “gain cells”) having amplifying driving transistors in pixel signal generators for generating pixel signals according to signal charge generated in charge generators. For example, many CMOS solid-state imaging devices have such a configuration.
In this type of amplifying solid-state imaging device, to read out pixel signals to an external source, address control is performed on a pixel portion in which a plurality of unit pixels are disposed so that the signals are selectively read from the individual unit pixels. That is, the amplifying solid-state imaging device is an example of an address-control solid-state imaging device.
For example, in an amplifying solid-state imaging device, which is one type of X-Y address solid-state imaging device having unit pixels disposed in a matrix, MOS-structured active devices (MOS transistors) are used for forming the pixels so that the pixels themselves have an amplifying function. That is, signal charge (photoelectrons) stored in photodiodes, which are photoelectric conversion devices, is amplified by the active devices and the amplified signal charge is read as image information.
In this type of X-Y address solid-state imaging device, for example, many pixel transistors are disposed in a two-dimensional matrix to form a pixel portion, the accumulation of signal charge in accordance with incident light in each line (row) or each pixel is started, and current or voltage signals based on the accumulated signal charge are sequentially read from the individual pixels by addressing. In MOS (including CMOS) solid-state imaging devices, an address control method for accessing the pixels in one row at one time and reading the pixel signals from the pixel portion in units of rows is mostly used.
The analog pixel signal read from the pixel portion is converted into digital data in an analog-to-digital converter (AD converter) if necessary. Accordingly, various AD conversion mechanisms have been proposed. In some of the known mechanism, in accordance with the method for accessing the pixels in one row at one time and reading the pixel signals from the pixel portion, a so-called column parallel system in which an AD converter and a signal processor for performing signal processing other than AD conversion are disposed for each vertical column is employed.
Various types of processing are executed on pixel signals output from the pixels to generate high-quality images or to use the pixel signals for special applications. Those types of processing largely include a first processing method for processing pixel signals in an analog area and then converting the pixel signals into digital data, and a second processing method for converting the analog pixel signals into digital data and then performing computation (digital computation) on the digital data.
For example, as the first processing method, the following mechanism for detecting edges is disclosed. Currents from a plurality of pixels for detecting light are simultaneously output to an output bus and are added or subtracted on the output bus. Then, the resulting currents are converted into pulse width signals having a magnitude in the time axis, and the pulse width signals are AD-converted by counting the numbers of clocks of the pulse widths of the pulse width signals in counter circuits disposed vertically in parallel with each other, thereby converting the addition/subtraction result into digital data. Also, a mechanism for detecting a moving part by generating the difference between pixel signals obtained at different time points in an analog area and by converting the difference into digital data (for example, binary values) is disclosed.
The following mechanism is known in the related art. By using the capacity within a pixel as an inter-pixel memory, signal charge detected by a photodiode is temporarily stored in the inter-pixel memory and is then read, thereby implementing an electronic shutter.
The following mechanism is known in the related art. By using the capacity within a pixel as the inter-pixel memory, the previous frame signal is stored and is added to the current frame signal in the pixel, thereby increasing the dynamic range, performing edge processing, or detecting a moving part.
As the second processing method, a mechanism for detecting a moving part by converting a plurality of analog video signals captured at different time points into digital data is known in the related art.
In terms of the circuit arrangement for performing computation processing, a method for performing computation processing outside the device (outside the chip) (off-chip method) is known in the related art.
A method for providing various processing functions, such as an addition/subtraction function, on the image sensor (such a technique is referred to as the “on-chip method”) is know in the related art. In particular, it is considered that a so-called “column parallel system” structure in which a signal processor is disposed in each vertical column for reading pixel signals from the pixel portion is suitable for the on-chip method.
However, in the above-described known mechanisms, a combination of the AD conversion and computation processing is not sufficient since it has advantages and disadvantages in terms of both the processing and the circuit arrangement. For example, the mechanism for performing addition/subtraction on signals in an analog area and then digitizing the resulting signals is not always efficient.
Additionally, some known AD conversion functions have a correlated double sampling function of removing noise components by performing subtraction processing. However, such a function merely performs difference processing between signal components and reset components in one pixel signal. That is, in this function, the difference between signal components and reset components having different physical properties in a signal output from the same unit element, such as a pixel, is obtained, and a plurality of signals, such as pixel signals, having the same physical property are not processed in this function. Accordingly, such processing is not computation performed between a plurality of pixels for generating high quality images or for using the pixel signals for special applications. To generate high quality images or to use the pixel signals for special applications, it is necessary to perform certain digital computation after AD conversion.